The iconic monument to Britain’s World War II prime minister in Parliament Square “was graffitied with red paint” overnight, the Metropolitan Police said on X.
“Officers were on scene within two minutes of being alerted shortly after 4am (0400 GMT),” the force said.
The incident came as police in Northern Ireland’s capital, Belfast, appealed for witnesses after a statue of Queen Victoria was vandalised, apparently by Irish Republican group Lasair Dhearg.
The group posted a picture on social media appearing to show one of its supporters pouring red paint over the statue of the monarch whom it called the “Famine Queen”, a reference to the Irish potato famine of the mid-19th century.
In London, the graffiti, which workers cleaned early Friday, called the wartime leader a “zionist war criminal”.
The words “free Palestine” and “stop the genocide” were also sprayed on the statue.
The man detained was also held on suspicion of supporting Palestine Action, a proscribed organisation under the Terrorism Act, police added.